Of the transducers or microphones in general use, most are of the capacitive, piezo-electrical or electro-dynamic type. Of these, transducers of the capacitive type are distinguished by their good sensitivity, their large bandwidth, their great stability and their low consumption and are generally used in hearing aids on account of these qualities.
These capacitive transducers conventionally comprise a diaphragm or membrane which is sensitive to acoustic pressure, and a rigid support plate. The diaphragm is disposed facing the support plate and is separated therefrom by an open space of small thickness. These two elements thus each form one of the two electrodes of a condenser. When the diaphragm moves in response to an acoustic pressure, the capacity of the condenser changes and this change is detected by a preamplifier, the inputs of which are connected to this diaphragm and to the rigid support plate respectively.
A large variety of constructions of these conventional transducers exist and they can be manufactured from numerous materials, preferably from a metallised or metallic film as the material constituting the diaphragm and from silicon as the material constituting the support plate.
A transducer of this type and a process for its manufacture are described in the publication entitled "Development of an electret microphone in silicon" by A. J. Sprenkels et al., in the journal Sensors and Actuators, 17(1989), pages 509-512.
According to this known process, a silicon substrate is used which is oxidized to form a layer of SiO.sub.2 on its upper and lower surfaces. The upper SiO.sub.2 surface is then etched to form cavities in the surface of the substrate. The substrate is then oxidized again, after which the lower layer of SiO.sub.2 is etched to form, on the one hand, air conduits each opening into one of the cavities provided on the upper surface and, on the other hand, passages adapted to fix the diaphragm and opening at the periphery of the cavities provided in the upper surface. The substrate is oxidized again and the diaphragm, composed of a sheet of Mylar.RTM. (PETP), is disposed on the upper surface of the substrate in such a way that it covers the cavities and the passages, and is fixed to the substrate using a polymer which is pulverised in the fixing passages. The upper electrode is then deposited by evaporation onto the sheet constituting the diaphragm, whereas the lower electrode is formed by the silicon of the substrate.
The manufacture of these transducers according to this process does, however, present a number of disadvantages.
The substantial differences in the nature of the various materials used (Mylar.RTM., film silicon, polymer) lead to problems of incompatibility with the semiconductor material micro-machining technologies used in the manufacture of these transducers. Also, according to this process, it is impossible to achieve reproducible characteristics between series of transducers derived from different silicon wafers. Also, this process does not make it possible to overcome the internal mechanical stresses present in the diaphragm and, consequently, does not permit reliable control of the sensitivity of the transducer.